Jetting discrete volumes of high viscosity liquid

ABSTRACT

In one aspect, an adhesive dispensing system  30  includes a dispensing module  32  having an inlet and an outlet, a supply  35  of hot melt adhesive maintained at low pressure, and a low pressure liquid passageway  44  communicating between the supply  35  and the inlet of the dispensing module  32 . Liquid material is received through the inlet at a low pressure, and the dispensing module  32  rapidly develops high pressure at the outlet  78  to jet the liquid material from the outlet  78 . In another aspect, an apparatus for jetting liquid material includes a dispenser body  38  having a liquid chamber  40 . A piston  54  is movably disposed within the liquid chamber  40 , from a position wherein a piston tip  58  is spaced from a complimentary shaped recess  76 , to a position wherein the piston tip  58  effectively seals off the recess  76 , then to a position wherein the piston tip  58  is received within the recess  76  to displace a discrete volume of liquid material from the recess  76.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalPatent Application Ser. No. 61/294,972, filed Jan. 14, 2010, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present invention relates generally to apparatus and methods fordispensing liquid materials, and more particularly to a jettingdispenser for jetting discrete volumes of high viscosity liquids.

BACKGROUND

Liquid dispensers for jetting relatively low viscosity fluids such assolder flux, conformal coatings, encapsulants, underfill material, andsurface mount adhesives, are known in the art and generally operate todispense small volumes of liquid material to a substrate by rapidlycontacting a valve seat with a valve member to create a distinct, highpressure pulse that ejects a small volume of liquid from the dispenser.As used herein, liquid material jetting refers to rapidly ejecting adiscrete mass of liquid material at a high velocity from a dispenser.Jetting is contrasted with extrusion wherein liquid material isdispensed as a continuous, elongate filament, generally referred to as a“bead” of adhesive. While drops can be formed by rapidly opening andclosing a valve during extrusion of liquid material, or by using air tobreak up an extruded bead as it is dispensed, these processes aredistinctly different from jetting processes wherein the discrete liquidmass is rapidly ejected directly from the dispenser at a high velocity.

FIGS. 1A and 1B illustrate the operation of a conventional jettingdispenser 10. In FIG. 1A, a valve member 12 is rapidly moved through afluid channel 14 in the direction of a valve seat 16 having an outlet18. As the valve member 12 approaches the valve seat 16, liquid material20 within the channel 14 flows around the valve tip 12 a. FIG. 1Bdepicts the jetting dispenser 10 at the instant the valve tip 12 acontacts valve seat 16. The momentum of the impact between valve member12 and valve seat 16 generates a pressure pulse that causes a smallvolume of liquid 20 a to be ejected through the outlet 18. U.S. Pat. No.5,747,102 to Smith et al. and U.S. Pat. No. 6,253,957 to Messerly etal., both assigned to the assignee of the present application, aredirected to jetting dispensers.

Conventional jetting dispensers require precise timing control to ensurethat a consistent volume of liquid material is jetted from thedispenser. For example, if the valve timing is too fast, there isinsufficient time for liquid material to refill within the dispenser,resulting in a lower than desired volume of liquid dispensed. Likewise,if the timing is too slow, the resulting volume of liquid is higher thandesired. It has long been thought that hot melt adhesive could not beadequately dispensed by a jetting dispenser, due to the high viscosityof hot melt adhesives and the differences in general rheology betweenhot melt adhesives and liquid materials that have been conventionallyused in jetting processes. Accordingly, hot melt adhesives havegenerally been dispensed by dedicated hot melt adhesive dispensingsystems that utilize high pressure to supply adhesive to a dispensingmodule. Typical pressures are in the range of 400 psi to 1000 psi. Avalve within the dispensing module is opened and closed to regulate theflow of the highly pressurized hot melt adhesive through an outletnozzle. There is a need for methods and apparatus for dispensing highlyviscous materials, such as hot melt adhesive, in discrete, small volumesand which overcome these and other drawbacks of conventional dispensingsystems.

SUMMARY

The present invention overcomes the foregoing and other shortcomings anddrawbacks of adhesive dispensing systems heretofore known for use indispensing small, discrete volumes of liquid material, particularlyhighly viscous liquid material such as hot melt adhesive. While theinvention will be described in connection with certain embodiments, itwill be understood that the invention is not limited to theseembodiments. On the contrary, the invention includes all alternatives,modifications and equivalents as may be included within the scope of thepresent invention.

In one aspect, a dispensing system for jetting liquid material includesan adhesive dispensing module having an inlet and an outlet, a supply ofhot melt adhesive maintained at low pressure, and a low pressure liquidpassageway communicating between the supply and the dispensing moduleinlet. The liquid material is received through the inlet at low pressureand the module rapidly develops high pressure to jet the liquid materialfrom the outlet.

In another aspect, the adhesive dispensing module includes a liquidchannel communicating with the outlet and a recess proximate the outlet.A piston is disposed in the channel, and the recess has a shape that iscomplementary to a tip of the piston. The high pressure that jets theliquid material from the outlet is developed as the piston tip is movedinto the recess.

In one embodiment, the supply of hot melt adhesive is adapted to containhot melt adhesive in solid form, and the dispensing system includes anauger for feeding the hot melt adhesive toward the dispensing module. Inanother embodiment, the dispensing system includes a vacuum feed devicefor feeding hot melt adhesive toward the dispensing module. In yetanother embodiment, the dispensing system includes first and secondadhesive dispensing modules having respective first and second inletsand first and second outlets. The first and second inlets receive liquidmaterial at the same low pressure from the supply through respectivefirst and second low pressure liquid passageways and the dispensingmodules generate high pressure proximate the respective outlets to jetliquid material therefrom at different rates.

In another aspect, an apparatus for jetting liquid material includes adispenser body having a liquid chamber that is couplable to a source ofliquid material and a piston having a piston tip is movably disposedwithin the liquid chamber. A recess communicating with the liquidchamber and a liquid outlet has a shape that is complimentary to theshape of the piston tip, whereby the piston tip may be received in therecess. The piston is movable from a position wherein the piston tip isspaced from the recess, to a position wherein the piston tip effectivelyseals of the recess, then to a position wherein the piston tip isreceived within the recess to displace a discrete volume of liquidmaterial from the recess.

In one embodiment, the apparatus further includes a nozzle that isoperatively coupled to an open end of the dispenser body, incommunication with the liquid chamber. The recess is formed in thenozzle. In another embodiment, the piston tip has a spherical shape.

In another aspect, a method for dispensing liquid material includessupplying the liquid material to a liquid chamber at a pressuresufficient to fill the liquid chamber, but not to dispense the liquidmaterial from an outlet associated with a recess and communicating withthe liquid chamber, effectively sealing off a discrete volume of liquidmaterial proximate the recess, and generating a high pressure at therecess to jet the discrete volume of liquid material from the outlet.

The above and other objects and advantages of the present inventionshall be made apparent from the accompanying drawings and thedescription thereof.

DESCRIPTION OF FIGURES

FIGS. 1A and 1B illustrate operation of a conventional jettingdispenser.

FIG. 2 is a perspective view of an exemplary dispensing system inaccordance with the present disclosure.

FIG. 3 is a cross-sectional view of the dispensing system of FIG. 2.

FIG. 4 is an enlarged detail view of the dispensing module of FIG. 3.

FIGS. 5A-5C are enlarged cross-sectional views illustrating operation ofthe dispensing system of FIG. 2.

FIG. 6 is a cross-sectional view similar to FIGS. 5A-5C, depicting asecond exemplary embodiment of a liquid dispensing system in accordancewith the present disclosure.

FIG. 7 is a schematic elevation view depicting a third exemplarydispensing system in accordance with the present disclosure.

FIG. 8 is a schematic elevation view of a fourth exemplary dispensingsystem in accordance with the present disclosure.

FIG. 9 is a schematic elevation view of a fifth exemplary dispensingsystem in accordance with the present disclosure.

FIG. 10 is a partial cross-sectional view of a sixth exemplarydispensing system in accordance with the present disclosure.

FIG. 11 is a schematic elevation view of a seventh exemplary dispensingsystem in accordance with the present disclosure.

FIGS. 12A-12C are enlarged cross-sectional views illustrating operationof another exemplary nozzle for use with the dispensing system of FIG.2.

FIG. 13A is a cross-sectional view of the nozzle 52A with the piston tipin the position shown in FIG. 12B.

FIG. 13B is a cross-sectional view of the nozzle 52A with piston tip inthe position shown in FIG. 12C.

DETAILED DESCRIPTION

FIG. 2 depicts an exemplary liquid dispensing system 30 for dispensingdiscrete, small volumes of highly viscous material 31, such as hot meltadhesive, to a substrate 33. For example, the dispensing system 30 maybe used to dispense liquid material having viscosities in the range ofabout 100 cps to about 20,000 cps. In another aspect, the jettingdispenser 10 may be used to dispense material having viscosities in therange of about 100 cps to about 25,000 cps. The dispensing system 30includes a liquid dispensing module 32 coupled to a source of liquidmaterial. In the embodiment shown, the module 32 is coupled to anadhesive manifold 34 for providing heated hot melt adhesive to themodule 32 at low pressure. For example, the pressure of the adhesivesupplied to the module 32 may be in the range of about 5 psi to about 40psi, or in other pressure ranges suitable for supplying hot meltadhesive to the module 32. The module 32 is also coupled to an airmanifold 36 for providing pressurized air from a source (not shown) tothe module 32.

Referring now to FIGS. 3 and 4, the dispensing module 32 includes amodule body 38 having a liquid chamber 40 formed therein. A liquidsupply passage 42 communicates with the liquid chamber 40 to supplyliquid material to the liquid chamber 40 from the adhesive manifold 34.In the embodiment shown, a liquid passageway 44 formed through the airmanifold 36 communicates with an outlet 46 of the adhesive manifold 34whereby liquid material flows from the outlet 46 of the adhesivemanifold 34, through the liquid supply passageway 44 and the supplypassage 42 of the module 32 to the liquid chamber 40. In the embodimentshown, manifold 34 includes a liquid reservoir 35 coupled to a pressuresource 37 for providing hot melt adhesive at low pressure (for example,in the range of about 5 psi to about 40 psi or other suitable pressureranges) through manifold passage 39 in communication with liquidpassageway 44. Because liquid material is provided at such a lowpressure, the dispensing system 30 does not require a piston pump, agear pump, or other types of pumps that are typically required todevelop high pressure. A simple diaphragm pump or pressure pot willsuffice to provide the low pressure. It will be appreciated that variousother arrangements and configurations may alternatively be used tosupply hot melt adhesive or other material to the module 32.

The module body 38 includes an open first end 50 communicating with theliquid chamber 40 and adapted to receive a dispensing nozzle 52. Thedispensing module 32 further includes a piston rod 54 having a first end56 reciprocatingly moveable within the liquid chamber 40. A piston tip58 is coupled to the first end 56 of the piston rod 54. While piston tip58 has been shown and described in this embodiment as a separatecomponent that is coupled to piston rod 54, piston tip 58 mayalternatively be integrally formed with the piston rod 54. The secondend 60 of the piston rod 54 is coupled to an air piston 62 that isslidably movable within a piston cavity 64 formed in the module body 38.Seals 66 a, 66 b disposed between the liquid chamber 40 and the pistoncavity 64 permit sliding movement of the piston rod 54 while sealing theliquid chamber 40 from the piston cavity 64. A compression spring 67biases seal 66 b against module body 38 to seal liquid chamber 40 whennozzle 52 is coupled to the first end 50 of the module body 38.Pressurized air from an air source (not shown), is provided to thepiston cavity 64 through air supply passages 68, 70 to rapidly move theair piston 62, and thus the piston rod 54 and the piston tip 58, indirections toward and away from the nozzle 52. In the embodiment shown,air supply passages 68, 70 are in fluid communication with air passages69, 71 in air manifold 36, which are in turn operatively coupled withthe air source. Pressurized air provided through air supply passage 68drives the piston in a direction away from nozzle 52, while pressurizedair provided through air supply passage 70 drives the piston in adirection toward nozzle 52. It will be appreciated that various othermethods and configurations for providing pressurized air to the pistoncavity 64 may alternatively be used. The module 32 further includes anadjustment knob 72 for selectively adjusting the stroke of the pistonrod 54 to facilitate varying the speed of the piston rod 58 during eachcycle of the piston rod 54. More stroke allows for greater accelerationand, therefore, higher velocity of the piston rod 54.

Nozzle 52 is coupled to the module body 38 at the open first end 50. Thenozzle 52 includes a nozzle body 74 having a recess 76 formed in a shapethat is complementary to the shape of the piston tip 58, whereby thepiston tip 58 may be received within the recess 76. In the embodimentshown, the piston tip 58 is semi-spherical, and the recess 76 has agenerally semi-spherical complementary shape. It will be appreciated,however, that nozzle tip 58 and recess 76 may have various othercomplementary shapes. The nozzle body 74 further includes an outlet 78communicating with the recess 76 via a nozzle passageway 80, wherebyliquid material in the liquid chamber 40 may be dispensed through thenozzle passageway 80 and nozzle outlet 78 when the piston tip 58 isreceived within the recess 76. Nozzle 52 may further include an O-ring81 for sealing against module body 38 at the open first end 50.

In operation, pressurized air is supplied to the piston cavity 64through the air supply passage 68 to cause the piston rod 54 to move ina direction away from the nozzle 52, such that the piston tip 58 iswithdrawn from the recess 76, as depicted generally in FIG. 5A, wherebyliquid material enters the liquid chamber 40 to fill the liquid chamber40 and the recess 76. Liquid material is supplied from the adhesivemanifold 34 at a pressure sufficient to fill the liquid chamber 40 andrecess 76, but not to cause liquid material to be dispensed from thenozzle outlet 78. Pressurized air is then supplied to the piston cavity64 through air supply passage 70 to cause the piston rod 54 to rapidlymove the piston tip 58 in a direction toward the nozzle 52. As thepiston tip 58 begins to enter the recess 76, the piston tip 58substantially seals the recess 76 along an upper edge 82 to define adiscrete volume of liquid between the piston tip 58 and the recess 76,as depicted in FIG. 5B. It will be appreciated that there is someclearance between the piston tip 58 and the upper edge 82 of the recess76 to permit the piston tip 58 to move into and out of the recess 76without binding against the recess 76. As used herein, substantiallysealing between the piston tip 58 and the recess 76 means that theclearance between the piston tip 58 and the upper edge 82 of the recess76 is sufficiently small that liquid material is forced by the pistontip 58 to be displaced through the nozzle passageway 80 and nozzleoutlet 78, rather than simply moving around the piston tip 58.

The piston rod 54 continues to move in a direction toward the nozzle 52such that the piston tip 58 continues to enter the recess 76 anddisplaces the liquid material in the recess 76 through the nozzlepassageway 80 and nozzle outlet 78, as depicted in FIG. 5C. Because thepiston tip 58 effectively seals recess 76 as described above, a discretevolume of liquid material is defined and a high pressure is developedbetween the piston tip 58 and the recess 76 as piston tip 58 continuesto enter the recess 76. The pressure generated may be in the range ofabout 100 psi to about 2500 psi. In another embodiment, the pressuregenerated may be in the range of about 400 psi to about 2500 psi. Inthis way, the discrete volume of liquid material 31 is jetted from thenozzle outlet 78 toward the substrate 33.

While module 32 has been described herein as having a piston rod 54 anda piston tip 58 driven by an air piston 62, it will be appreciated thatvarious other structure and methods may alternatively be used to developa high pressure near a nozzle outlet and to jet a discrete volume ofliquid material therefrom.

After the discrete volume of liquid material 31 is jetted from thenozzle outlet 78, as described above, pressurized air may again besupplied to piston cavity 64 to cause piston rod 54 to move in adirection away from nozzle 52 and the entire process may be repeated todispense successive discrete volumes of liquid material from nozzleoutlet 78 as may be desired. It will be appreciated that operation ofthe dispensing system 30 as described above may be controlled by acontroller to dispense the discrete volumes of liquid material at adesired frequency relative to the speed of a substrate 33 movingrelative to the module 32 to produce a desired spacing between thediscrete volumes of liquid material.

While operation of the module 32 has been described and illustrated in amanner wherein piston tip 58 becomes fully seated within recess 76 todispense substantially the entire volume of liquid material withinrecess 76, it will be appreciated that the range of motion of piston 54may alternatively be controlled such that piston tip 58 is not fullyseated within recess 76 at the end of each stroke of piston 54, wherebyan amount less than the entire volume of liquid material within recess76 may be dispensed.

FIG. 6 depicts another exemplary embodiment of a liquid dispensingsystem similar to the liquid dispensing system 30 described above, butwherein a modified dispensing nozzle 52 a is coupled to the module body38. The nozzle body 74 a shown in this embodiment is configured to jetdiscrete volumes of liquid material through outlet 78 in a directionsubstantially perpendicular to the direction of movement of the pistonrod 54. Operation of the dispensing system is otherwise similar to thatdescribed above with respect to FIGS. 2-5C and similar features aresimilarly numbered. Such an embodiment may be useful, for example, forjetting discrete volumes of liquid material to vertically orientedsurfaces, such as the surfaces of labels or containers during labelingapplications. While nozzle body 74 a has been shown and described ashaving an outlet 78 configured to jet liquid material in a directionsubstantially perpendicular to the direction of movement of the pistonrod 54, it will be appreciated that various other configurations ofnozzle bodies may be used to jet liquid material in various otherdirections, or in various other manners, as may be desired for a givenapplication.

FIG. 7 depicts another exemplary liquid dispensing system 100 fordispensing discrete, small volumes of highly viscous material 102, suchas hot melt adhesive, to a substrate 104. The dispensing system 100includes a dispensing module 106, as described above, operativelycoupled to a support structure 108 for movement relative to thesubstrate 104. Because dispensing module 106 is constructed in a mannersimilar to module 32 described above, the details of module 106 are notrepeated here. The dispensing system 100 further includes a pressure pot110 containing a supply of hot melt adhesive and configured to providethe hot melt adhesive to the dispensing module 106 at a low pressure.The pressure pot 110 may be adapted to melt and/or maintain a heatedtemperature of the hot melt adhesive. The pressure pot 110 is coupled toa source of low pressure air 112, such as by a hose 114 or otherappropriate connection. The pressure pot 110 further includes a gauge116 for indicating the internal pressure of the pressure pot 110. Hotmelt adhesive is transferred between the pressure pot 110 and thedispensing module 106 by a low pressure liquid passageway 118. The lowpressure liquid passageway 118 may be a flexible hose, a rigidconnection such as pipe, or any other suitable structure. Pressurizedair from an air source 120 is also provided to the dispensing module 106to operate the dispensing module 106 as described above. Specifically,the pressurized air is used to rapidly move a piston to generate highpressure at an outlet of a nozzle for jetting liquid material as adiscrete, small volume toward the substrate in a manner similar to thatdescribed above with respect to FIGS. 5A-5C.

FIG. 8 depicts another exemplary liquid dispensing system 130 fordispensing discrete, small volumes of highly viscous material, such ashot melt adhesive, to a substrate. In this embodiment, first, second,and third dispensing modules 132 a, 132 b, 132 c are coupled to a commonpressure pot 134 by respective low pressure liquid passageways 136 a,136 b, 136 c, such as flexible hoses, rigid pipe, or any other suitablestructure. The pressure pot 134 may be adapted to melt and/or maintain aheated temperature of the hot melt adhesive. Each module 132 a, 132 b,132 c is operatively coupled to a high pressure source 138 a, 138 b, 138c for operating the respective dispensing modules 132 a, 132 b, 132 c,in a manner as described above. The modules 132 a, 132 b, 132 c may becoupled to the same pressure source, or individual pressure sources 138a, 138 b, 138 c may be dedicated to the respective modules 132 a, 132 b,132 c. In use, liquid material, such as hot melt adhesive, is suppliedfrom the pressure pot 134 at a common pressure through the respectivelow pressure liquid passageways 136 a, 136 b, 136 c to the respectivedispensing modules 132 a, 132 b, 132 c. Thereafter, each dispensingmodule 132 a, 132 b, 132 c is operated to dispense discrete, smallvolumes of the liquid material to a respective substrate 140 a, 140 b,140 c. The modules 132 a, 132 b, 132 c may be operated independently ofone another such that the volume and frequency of dispensing thediscrete volumes of liquid material may be different from module tomodule. For example, as depicted in FIG. 8, the first module 132 a maydispense liquid material at a first discrete volume 142 a and at a firstfrequency relative to the movement of the first substrate, such that thedispensed volumes 142 a of liquid material are received on the substrate140 a in a closely spaced arrangement. The second dispensing module 132b may be operated to dispense a second discrete volume 142 b of liquidmaterial, larger than the first discrete volume 142 a dispensed from thefirst dispenser 132 a, and at a second frequency relative to themovement of the second substrate 140 b such that the discrete volumes142 b of liquid material are received on the substrate 140 b in a spacedarrangement. The third dispensing module 132 c may be operated todispense a third discrete volume 142 c of liquid material, smaller thanthe first and second discrete volumes 142 a, 142 b of liquid materialand at a frequency relative to the movement of the third substrate 140 csuch that the dispensed third discrete volumes 142 c of liquid materialare spaced apart on the substrate 140 c at a desired distance.

FIG. 9 depicts yet another exemplary liquid dispensing system 150 fordispensing discrete, small volumes of viscous material 152 to asubstrate 154. In this embodiment, the supply 156 for providing liquidmaterial, such as hot melt adhesive, is directly coupled to and carriedby the dispensing module 158. Because the dispensing module 158 isconfigured to dispense such small volumes of liquid material 152, thesize of the adhesive supply 156 may be selected to contain only enoughliquid material required to dispense during a given time period, such asan entire shift. The adhesive supply 156 comprises a housing 160,defining a reservoir 162 for receiving material to be dispensed, such ashot melt adhesive 163, in solid or particulate form, and a closure 164selectively positionable over the reservoir 162. The reservoir 162 isconfigured to melt and/or maintain a heated temperature of the hot meltadhesive. Appropriate passageways 166 or other fluid connections providefluid communication between the reservoir 162 and the dispensing module158. Module 158 is operatively coupled to a source of pressurized air168, and operates as described above. If the dispensing system 150 ispressurized to transfer adhesive from the reservoir 162 to thedispensing module 158, the closure 164 may be configured to seal thereservoir 162, at least to withstand the low pressure needed to transferthe hot melt adhesive from the reservoir 162 to the dispensing module158.

FIG. 10 depicts yet another exemplary liquid dispensing system 170 fordispensing discrete, small volumes of highly viscous material 172 to asubstrate 174, similar to the dispensing system shown 130 and describedabove with respect to FIG. 9, but wherein the adhesive supply 176includes a hopper 178 for receiving hot melt adhesive 180 in solid,particulate form. The adhesive supply 176 further includes an auger 182driven by a drive shaft 184 for transferring the particulate hot meltadhesive 180 from the hopper 178 to a manifold 186 for melting the hotmelt adhesive and subsequently providing the melted hot melt adhesive tothe dispensing module 188. In this embodiment, the supply pressure ofthe hot melt adhesive is provided by the auger 182. The dispensingmodule 188 is coupled to a source of pressurized air 190 for operatingthe dispensing module 188, and the dispensing module 188 otherwiseoperates as discussed above.

FIG. 11 depicts another exemplary liquid dispensing system 200 fordispensing discrete, small volumes of highly viscous material 202, suchas hot melt adhesive, to a substrate 204. The dispensing system 200includes a manifold 206 and dispensing module 208 coupled to a source ofpressurized air 210 for operating the dispensing module 208 as describedabove. Material 212 to be dispensed, such as hot melt adhesive, issupplied to the manifold 206 in particulate form from a tank 214 that isoperatively coupled to an inlet 216 of the manifold 206 by a passageway218. In this embodiment, the tank 214 is unpressurized and material 212is drawn through an inlet 220 of a vacuum feed device 222 andtransported through the passageway 218 to the manifold 206, whereafterthe material is melted and directed to the dispensing module 208 in amanner similar to that described above. Manifold 206 is suitablyconfigured to melt the material 212 and to supply the melted material212 to module 208 at low pressure. The tank 214 may include a closure224 for covering a reservoir 226 holding the liquid material 212, andwheels 228 to facilitate moving the tank 214. In use, high pressure isdeveloped within the dispensing module 208 to jet a small, discretevolume of the liquid material 202 to the substrate 204 moving beneaththe module 208.

FIGS. 12A-12C depict another exemplary embodiment of a nozzle 52 acoupled to a module body 32 a and including a nozzle body 74 a inaccordance with the principles of the present disclosure, whereinsimilar reference characters represent similar features. In thisembodiment, nozzle body 74 a includes a plurality of lobes 230 adjacentliquid chamber 40 a to help guide piston tip 58 a on piston rod 54 a asthe piston tip 58 a is moved into and out of recess 76 a in a mannersimilar to that described above with respect to FIGS. 5A-5C. When pistontip 58 a is withdrawn from recess 76 a (FIG. 12A), low pressure liquidmaterial can flow into recess 76 through the lobes 230. As piston tip 58a enters recess 76 a (FIG. 12B), piston tip 58 a substantially seals therecess 76 a in a manner similar to that described above. As piston tip58 a continues to move in a direction toward nozzle 52 a, liquidmaterial in the recess 76 a is displaced though the nozzle passageway 80a and outlet 78 a. When piston tip 58 a is fully seated against recess76 a (FIG. 12C), the piston tip 58 a prevents low pressure liquidmaterial from “drooling” from the nozzle outlet 78 a. FIG. 13A is across-sectional view of the nozzle 52A with piston tip 58 a in theposition shown in FIG. 12B. FIG. 13B is a cross-sectional view of thenozzle 52A with piston tip 58 a in the position shown in FIG. 12C.

While various aspects in accordance with the principles of the inventionhave been illustrated by the description of various embodiments, andwhile the embodiments have been described in considerable detail, theyare not intended to restrict or in any way limit the scope of theinvention to such detail. The various features shown and describedherein may be used alone or in any combination. Additional advantagesand modifications will readily appear to those skilled in the art. Theinvention in its broader aspects is therefore not limited to thespecific details, representative apparatus and methods and illustrativeexamples shown and described. Accordingly, departures may be made fromsuch details without departing from the scope of the general inventiveconcept.

What is claimed is:
 1. An adhesive dispensing system, the systemcomprising: an adhesive dispensing module having an inlet and an outlet,said adhesive dispensing module receiving liquid material through saidinlet at low pressure and rapidly developing high pressure at saidoutlet to thereby jet the liquid material therefrom; a supply of hotmelt adhesive maintained at low pressure; a low pressure liquidpassageway communicating between said adhesive dispensing module inletand said supply; a liquid chamber communicating with said outlet; arecess proximate said outlet; and a piston tip disposed in said liquidchamber, wherein: said recess is shaped complementary to said piston tipand adapted to receive said piston tip therein, said piston tip developssaid high pressure when said piston tip is moved into said recess, andsaid piston tip is reciprocatingly movable within said liquid chamberbetween a first position wherein hot melt adhesive from said supply isadmitted to said recess, a second position wherein said piston tip sealsthe liquid chamber from said supply of hot melt adhesive, and a thirdposition wherein hot melt adhesive is jetted from said outlet under highpressure.
 2. The adhesive dispensing system of claim 1, wherein saidsupply of hot melt adhesive is adapted to contain hot melt adhesive insolid form.
 3. The adhesive dispensing system of claim 2, furthercomprising an auger for feeding hot melt adhesive toward said adhesivedispensing module.
 4. The adhesive dispensing system of claim 2, furthercomprising a vacuum feed device for feeding hot melt adhesive towardsaid adhesive dispensing module.
 5. The adhesive dispensing system ofclaim 1, further comprising first and second adhesive dispensing moduleshaving respective first and second inlets and first and second outlets,said first and second inlets receiving liquid material at the same lowpressure from said supply through respective first and second lowpressure liquid passageways and generating high pressure proximate saidrespective outlets to jet liquid material therefrom at different rates.6. The adhesive dispensing system of claim 1, wherein said supply issupported on said adhesive dispensing module.
 7. An apparatus forjetting liquid material, the apparatus comprising: a dispenser bodycouplable to a source of liquid material, said dispenser body includinga liquid chamber; a piston movably disposed within said liquid chamberand having a piston tip; and a recess in fluid communication with saidliquid chamber and with a liquid outlet, said recess shapedcomplimentary to said piston tip such that said piston tip is receivablewithin said recess, wherein: said piston tip develops a high pressurewhen said piston tip is moved into said recess, and said piston isreciprocatingly movable from a first position wherein said piston tip isspaced from said recess for liquid material from said source of liquidmaterial to be admitted to said recess, to a second position for saidpiston tip to effectively seal off said recess from said source ofliquid material to define a discrete volume between said piston tip andsaid recess, and then to a third position wherein said piston tip isreceived within said recess to displace said discrete volume from saidrecess through said liquid outlet for said liquid material to jet fromsaid outlet under said high pressure.
 8. The apparatus of claim 7,further comprising: an open end in said dispenser body, said open endcommunicating with said liquid chamber; and a nozzle operatively coupledto said dispenser body at said open end, wherein said recess beingformed in said nozzle.
 9. The apparatus of claim 7, wherein said pistontip has a spherical shape.
 10. A method of dispensing liquid material,the method comprising: supplying liquid material to a liquid chamber ata pressure sufficient to fill the liquid chamber, but not to dispensethe liquid material from an outlet associated with a recess andcommunicating with the liquid chamber; effectively sealing off adiscrete volume of liquid material proximate the recess by a piston tipdisposed in the liquid chamber, wherein the recess is shapedcomplementary to the piston tip and adapted to receive the piston tiptherein; and generating a high pressure at the recess to jet thediscrete volume of liquid material from the outlet when the piston tipis moved into said recess, wherein hot melt adhesive is jetted from saidoutlet under said high pressure.
 11. The method of claim 10, wherein thehigh pressure generated is in the range of about 400 psi to about 2500psi.